The following application is hereby incorporated by reference herein: U.S. patent application Ser. No. 12/367,243 (“the '243 application”), filed Feb. 6, 2009.
Potentiometers are electric devices used in a variety of electrical circuits, including those where a specific voltage output is needed. Potentiometers allow for a user to create a constant resistance between the terminals, whereupon the user can change the resistance between the terminals by mechanically adjusting the potentiometer. In a digital potentiometer, a digital input code is input to the potentiometer which accepts the input code and adjusts the resistance of the potentiometer accordingly.
A digital potentiometer has three terminals: two primary terminals and a third terminal referred to as the wiper. The resistance between the primary terminals is constant and is equal to a total end-to-end resistance of the entire potentiometer. The resistance between the first primary terminal, A, and the wiper is equal to:
                                          D            *                          R              TOTAL                                            2            n                          ,                            (        i        )            wherein D is a decimal equivalent of an n-bit input code, RTOTAL is a total end-to-end resistance of the entire potentiometer, and n is the number of bits of the input code to the potentiometer.
Conversely, the resistance between the second primary terminal, B, and the wiper is equal to:
                                                        R              TOTAL                        *                          (                                                2                  n                                -                D                            )                                            2            n                          ,                            (        ii        )            wherein the total resistance between terminals A and B is the total end-to-end resistance of the potentiometer and is equal to:
                                          D            *                          R              TOTAL                                            2            n                          +                                                            R                TOTAL                            *                              (                                                      2                    n                                    -                  D                                )                                                    2              n                                .                                    (        iii        )            
The problem with traditional digital potentiometers is that the resistance between terminal A and the wiper (one of the resistance “arms”) is dependant on the resistance between terminal B and the wiper (the other resistance “arm”). In typical architectures for the digital potentiometer, the primary terminals share a final string array at the wiper. Therefore, any adjustment of the resistance between one of the terminals and the wiper changes the resistance between the other terminal and the wiper, because of the presence of the single shared string array. This problem is further evidenced by equations (i) and (ii) in which a change to the input code D, changes the resistance for each of the resistance arms. In traditional digital potentiometers, a single digital input is provided to each of the primary terminals, and therefore the resistance of the resistance arms is dependent on the single digital input.
The current structure of a traditional digital potentiometer only allows for a selection of the terminal A-to-wiper resistance that is dependent and based on set ratios to the terminal B-to-wiper resistance, because of the presence of a shared string array between each of the terminals and the wiper terminal. Therefore, there remains a need in the art, for a digital potentiometer architecture which allows for the independent control of the resistance between each of the primary terminals and the wiper.